He says it's most commonly used Linux distro.
He didn't say that most commonly Linux Server distro is Ubuntu.

Ahh sorry. The line I quoted said he was using Ubuntu Server - which isn't the same as Desktop Ubuntu.

Reading through it again, it's become quite clear that he's just using regular Ubuntu. Very poor choice of distro in my opinion (given the point behind cluster servers) but at least he's been detailed enough where people could translate the same experience to other Linuxes.